This invention relates to a musical tone forming device in an electronic musical instrument, which is simple in arrangement and is capable of forming a variety of musical tone signals, and more particularly to a device which utilizes a frequency modulation (FM) technologies to form musical tone signals.
A musical tone forming device according to the FM technology is described in the specification of U.S. Pat. No. 4,018,121. In order to provide richer musical tones with such a device utilizing the frequency modulation, it is preferable to employ various composition systems such as "a multi-series system", "a multi-term system" and "a multiple system".
In the "multi-series system", a plurality of frequency-modulated waveforms are separately (individually) calculated (obtained by calculation) in response to key depression, and the resultant respective waveform data are added together to provide a musical tone signal. One example of the calculation formula is as follows: ##EQU1## where A.sub.i (i=1, 2, . . . n) are the coefficients to determine the amplitudes of respective waveforms to be calculated, I.sub.i are the coefficients to determine the depths of the respective modulation, .omega..sub.ci and .omega..sub.mi are the angular velocities to determine the frequencies of the carrier waves and the modulating waves, respectively, and correspond to the tone pitch (frequency) of the depressed key, and Z and G are the predetermined mathematical functions such as sinusoidal functions (sin).
In the "multi-term system", a signal waveform having a frequency corresponding to the depressed key is frequency-modulated with a plurality of signal waveforms, to calculate (produce by calculation) a musical tone signal waveform. The calculation formula is as follows: ##EQU2## where A.sub.1, .omega..sub.c1 and .omega..sub.mi are identical with those in the calculation formula (1).
In the "multiple system", the signal waveform of a frequency corresponding to a depressed key is frequency-modulated, in a multiple mode (multi-times), with a plurality of signal waveforms, to calculate a musical tone signal waveform. One example of a calculation formula which is employed in the case where, for instance, the signal waveform of a frequency corresponding to a depressed key is subjected to frequency modulation in a double mode, is as follows: EQU F=A.sub.1 .multidot.Z[.omega..sub.c1 t+I.sub.1 .multidot.G{.omega..sub.m1 t+A.sub.2 .multidot.Z(.omega..sub.c2 t+I.sub.2 .multidot.G(.omega..sub.m2 t)}] (3)
where the values A.sub.1, A.sub.2, I.sub.1, I.sub.2, .omega..sub.c1, .omega..sub.c2, .omega..sub.m1 and .omega..sub.m2 are identical with those defined above.
Musical tone signals provided according to the above-described various composition systems are different in harmonic spectrum, and accordingly musical tones different in tone color can be formed. These systems have tone colors peculiar thereto. Accordingly, if the systems are switched according to a tone color selected or desired, produced musical tones can be desirably made richer. This may be achieved by a method in which musical tone generating devices according to the above-described various composition systems are juxtaposed and are selectively operated.